The present invention concerns quantitative imaging or gas emissions utilizing optical techniques. It is of great interest to be able to detect and quantify gas flows. Leakage of hydrocarbons from oil rigs, petrochemical industry, tank farms or natural gas pipelines constitute an economical loss, an environmental problem and a safety concern. Releases in connection with accidents involving transport vehicles or in industrial operations need to be controlled. Natural emissions of green-house gases from geophysical sources, e.g. volcanoes, geothermal plants or marshes or bogs need to be chartered. In the indoor environment it is important to be able to control the functioning of a ventilation system or an air extraction channel. It would be of great interest if the gas releases could be visualized and quantified in near real time, since the remedy actions in many cases need to be launched immediately. Gas imaging can be performed based on the selective absorption of optical radiation. If an artificial light source is used we talk about active techniques while the utilization of the background radiation is referred to as passive techniques. The infrared thermal background radiation is of special interest for passive imaging. In a non-limiting embodiment of the invention this radiation is used for quantitative imaging of flowing hydrocarbons.
Molecular gases exhibit characteristic absorption lines in the visible and ultra-violet spectral region (electronic transitions) and in the infrared spectral region (rotational-vibrational transitions). The presence of gas in the atmosphere is manifested through absorption of a transmitted beam at specific wavelengths. The well-known DOAS (differential Absorption Optical Spectroscopy) technique as well as the Fourier-Transform Spectrometry principle utilizes light from a continuos light source, with the beam transmitted over an atmospheric path ending at the receiver. Alternatively, the sky radiation can be utilized, which is the case also when correlation techniques are used (COSPEC techniques with a mechanical mask arrangement in the image plane of the spectrometer, or gas correlation techniques with a gas cell intermittently introduced in the light pathway in front of the detector). Tuneable semiconductor lasers can also be utilized for absorption spectroscopy of molecular gases. Pulsed laser systems allow range-resolved monitoring of gas clouds utilizing differential absorption lidar (LIght Detection And Ranging).
The techniques mentioned above deal with measurements over given paths. However, imaging of the extension of a gas cloud is important in many cases. This can be achieved with imaging lidar techniques. A more simple approach is to utilize passive techniques. This has been achieved in the infrared spectral region utilizing a xe2x80x9cheat cameraxe2x80x9d, which has been equipped with a band pass filter for the absorption region of the particular gas. If a sufficient amount of gas is present it can be visualized with a lower intensity than the surrounding. The fact that many gases, such as hydrocarbons, absorb in the same wavelength range, constitutes a problem. The gas correlation technique then provides an automatic discrimination between the gases as recently has been reported. Two images are then recorded, a direct one and one through a gas cell, containing the particular kind of gas to be imaged, in an optically thick concentration. The gas completely blocks the gas of interest and provides a reference image. However, the direct image is influenced by the specific gas absorption. By subtraction or division of the images the gas is enhanced, and the surrounding areas are eliminated. Other gases with absorption lines not matching the gas filter are also eliminated by this image processing. Until recently, gas correlation techniques needed to utilize an artificially heated background surface in order to reach a sufficient level of thermal radiation.